Bad lag after reformatting hard drive

i have recently had to change my hard drive after i had problems with rebooting my pc. I have inserted a completely new hard drive and loaded a new windows XP operating system. The computer loads up fine but gets bad lag spikes when there are sounds playing along with other stuff running ie internet explorer and the sound still sometimes lags even if the system is idle. i have downloaded all the up to date drivers for the sound card, cd rom and motherboard but i still get the lag spikes, also the cpu usage when idle is still running at around 15% and spikes to around 50-60% when i do something small like open my documents.
I seem to have tried everything but still cant get to the bottom of this, could anyone please help?